This disclosure relates to providing natural hear-through in active noise reducing (ANR) headphones, reproducing audio signals simultaneously with hear-through in ANR headphones, and eliminating the occlusion effect in ANR headphones.
Noise reducing headphones are used to block ambient noise from reaching the ear of a user. Noise reducing headphones may be active, i.e., ANR headphones, in which electronic circuits are used to generate anti-noise signals that destructively interfere with ambient sound to cancel it, or they may be passive, in which the headphones physically block and attenuate ambient sound. Most active headphones also include passive noise reduction measures. Headphones used for communications or for listening to entertainment audio may include either or both active and passive noise reduction capabilities. ANR headphones may use the same speakers for audio (by which we include both communications and entertainment) and cancellation, or they may have separate speakers for each.
Some headphones offer a feature commonly called “talk-through” or “monitor,” in which external microphones are used to detect external sounds that the user might want to hear. Those sounds are reproduced by speakers inside the headphones. In ANR headphones with a talk-through feature, the speakers used for talk-through may be the same speakers used for noise cancellation, or they may be additional speakers. The external microphones may also be used for feed-forward active noise cancellation, for picking up the user's own voice for communications purposes, or they may be dedicated to providing talk-through. Typical talk-through systems apply only minimal signal processing to the external signal, and we refer to these as “direct talk-through” systems. Sometimes direct talk-through systems use a band-pass filter to restrict the external sounds to voice-band or some other band of interest. The direct talk-through feature may be manually triggered or may be triggered by detection of a sound of interest, such as voice or an alarm.
Some ANR headphones include a feature to temporarily mute the noise cancellation so that the user can hear the environment, but they do not simultaneously provide talk-through, rather, they rely on enough sound passively passing through the headphones to make the environment audible. We refer to this feature as passive monitoring.